ANALYSIS OF ANISOTROPIC DISPLACEMENT PARAMETERS IN A SERINE PROTEASE Th.R. Schneider, European Molecular Biology Laboratory (EMBL), c/o DESY, Notkestr. 85, 22607 Hamburg, F. Parak, Faculty of Physics, E17 of the Technical University Munich, 85748 Garching

Based on X-ray diffraction data to atomic resolution the structure of a serine protease has been determined at 90, 120, 180 and 300 K. Refinements were performed employing restrained anisotropic displacement parameters (ADP's) as implemented in the program SHELXL93¹. After convergence several cycles of block-matrix least-squares refinement were run to determine estimated standard deviations for coordinates and displacement parameters. Graphical inspection of vibrational ellipsoids revealed that significant anisotropy in atomic displacements can frequently be found for solvent exposed sidechains and carbonyl oxygens. Numerical analysis of the ADP's originated from the evaluation of the rigid-body test² whereby extended parts of the polypeptide backbone were detected as 'potentially rigid'. Fits of physically constrained TLS-models to these regions allow the explanation of parts of the individual atomic displacements in the protein molecule by a temperature dependent disorder of the molecule as a whole. It must be emphasized that, since internal normal modes are not considered in a pure TLS-analysis, the derived magnitudes for the TLS-terms only represent upper limits to the real values3. The remaining contributions to the ADP's can be linked to conformational substates, internal dynamics and zero-point motions.

¹G.M. Sheldrick; SHELXL-93; University of Goettingen (1993)

²R.E. Rosenfeld et al.; Acta Cryst. A34:828-829 (1978)

³A. Kidera and N. Go; J.Mol.Biol. 225:457-475 (1992)